CHARA - STRUCTURE OF VEGETATIVE BODY AND REPRODUCTION

A. CLASSIFICATION:

Division – Chlorophyta

        Class – Chlorophyceae

                   Order – Charales

                             Family – Characeae

                                      Genus – Chara

The genus Chara comprises about 180 species. It is commonly known as “stone-wort”. It is a submerged aquatic alga of fresh water ponds, lakes, tanks, or slow flowing streams. The algae remains attached to the sandy or muddy bottoms with the help of rhizoids.

About 30 species of Chara have been identified from India. The most common Indian species are – C. handae, C. hatei, C. nuda, C. grovesii, and C. pashanii, etc.

B. STRUCTURE OF THE THALLUS:

The individual plant generally attains al length of 20 to 30 cms. The thallus has a long slender, upright main axis which is differentiated into a well marked nodes and internodes. The internode consists of a single, elongated multinucleate and cylindrical cell. From each of the node arise the following four types of appendages –

(a) Branchlets:- From each node arise a whorl of short branches of limited growth, called branchlets. They are also called as primary laterals or leaves. They alternate with one another at the successive nodes. The branchlets consist of limited number of nodes and internodeswhich is the characteristic of a particular species.

(b) Long Branches:- In addition, the stem node may bear one or more branches of unlimited growth. They are also known as axillary branches. They usually arise singly at some of the older nodes of the main axis on the inner side of the oldest primary lateral in a whorl.

(c) Stipulodes:- These are unicellular outgrowths that arise from the basal node of each branchlet. In the majority of species of Chara there are two stipulodes at the base of each branchlet, one on either side. They are arranged in a single row or circle at the stem node.

(d) Cortex:- In many species the intermodal cell is covered by a sheath of vertically elongated narrow cells constituting the cortex. Such species are described as corticated (C. zeylanica) and others ecorticated (C. wallichii).

C. REPRODUCTION: Chara  reproduces both by vegetative and sexual methods of reproduction.

1. Vegetative Reproduction:- The common methods vegetative reproduction are -

(a) Amylum stars:- These are star shaped aggregations of cells developed on the lower nodes of the main axis. The cells of the amylum stars are laden with amylum starch. When detached, the amylum stars grows into a new plant.

(b) Bulbils:- These are small, rounded tuber-like structures developed upon the rhizoids or stem nodes. The bulbils are perennating bodies which carry on vegetative propagation. The detached bulbils form the new plant.

(c) Amorphous bulbils:- These are clumps of several small cells laden with food materials. They are produced on the lowest stem nodes or nodes of the rhizoids as lateral outgrowths.

(d) Secondary protonema:- Sometimes naked adventitious protonema-like branches arise from the surviving nodes of the older plants. They may also develop from the primary rhizoid ring or dormant apices. The secondary protonema gives rise to new plant like primary protonema.

2. Sexual Reproduction:- Sexual reproduction in Chara is advanced oogamous type. The majority of the species of Chara are monoecious, i.e., homothallic bearing both male and female sex organs. The male reproductive structure or antheridia is known as globule and the female or oogonia is called nucule.

(a) Male Reproductive Structure of Globule:- A mature globule is more or less globular, large and stalked structure, bright yellow or red in colour.

The wall of the globule is composed of 8 curved plates called shield cells. Each shield cell bears in its centre a rod-shaped radially elongated cell called manubrium or handle cell. From the distal end of the manubrium arise 8 centrally placed rounded cells called primary capitula. Each primary capitulum again produce 6 secondary capitula and therefore the total number of secondary capitula is 48. From each secondary capitulum, a long whip-like branched or unbranched thread, called antheridial filaments develop. Each antheridial filament bears a row of 100-200 cells which functions as sperm mother cells and gives rise to single uninucleate, spirally coiled, elongated, biflagellated antherozoid or sperm.

When the sperm matures, they get liberated from the antheridial filament in the surrounding water by the gelatinization of the sperm mother cells.

(b) Female Reproductive Structure of Nucule:- The mature female reproductive structure or nucule is oval or elliptical structure attached to the node with the help of pedicel cell or stalk cell. The nucule remains enveloped by 5 long spirally twisted tube cells which gives the twisted appearance to the enclosing oogonium. Each of these tube cells terminated into a small erect cell at the apex of the nucule, called the coronary cell. The coronary cells form the corona of the nucule.

The oogonium consists of a single, large, uninucleate oosphere or ovum densely filled with oil globules and starch grains. The mature egg contains a single nucleus situated towards the base of the oogonium.

When the nucule attains maturity, the spirally twisted tube cells separate from one another just below the corona to make the small, narrow aperture.

(c) Fertilization:- The antherozoids after liberation from the antheridial filament swims in water and collect near the corona, then the free swimming antherozoids enter the nucule through the narrow slit openings formed just near the corona cells. One of the antherozoid traverses down towards the base of the oogonium, penetrates the ovum, comes in contact with the egg nucleus and fuses to form the diploid zygote.

The zygote is hard and nut-like structure of different colours ranging from red or brown or light yellow and also black in different species. The zygote nucleus (2n) comes towards the upper side, comes out by rupturing the wall and settles at the bottom of the ponds. Zygote germinates after a period of rest.

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